Dagmar Heim

Swiss Federal Veterinary Office, Bern, Switzerland

During 1998, a total of 1719 cattle brains were examined by histology and immunohistochemistry and tested with the Prionics-Check-Test -- a Western blot test. Five brains were found to be test-positive.

The objective of the above study was to get an appreciation of the risk of herdmates of BSE-cases and to validate the Prionics-Check-Test. The test results showed high agreement with those obtained by immunohistochemistry.

Therefore, it was decided to use the test on 3000 routinely slaughtered cattle so as to evaluate its performance under practical conditions of time-pressure and high throughput. 3022 cattle, 90% over two years of age, were sampled at slaughter, tested for PrP accumulations in the brain using the Prionics-Check-Test and examined by histology and/or immunohistochemistry. One 4-year-old animal was found to be positive to the test and confirmed for BSE by means of immuno-histochemistry. This study showed that the Prionics-Check-Test performs satisfactorily under field conditions; i.e., with large numbers of samples being processed and independent on the tissue quality. The results were available within 6-8 hours.

Furthermore, it was evident that sampling the brainstem through the foramen magnum with a special spoon is easy to perform and a good method to achieve samples with a high quality.

Mandatory reporting of clinically suspect animals alone accounts for only part of the BSE cases in the population. Preclinical cases, animals in the early clinical stages of the disease and animals in which the signs are misinterpreted, remain undetected and may be routinely slaughtered or culled for reasons other than BSE.

Encouraged by the results of the studies in 1998, Switzerland initiated a targeted surveillance programme in January 1999 to enhance detection of BSE cases in all segments of the adult cattle population and to complement detection through mandatory reporting of clinically suspect animals. Because the youngest animal diagnosed with BSE in Switzerland was 32 months old, the target population was cattle with at least four permanent incisor teeth; i.e., over 24 months of age. In studies of experimentally infected cattle, detectable PrP accumulations were not reached earlier than 28 months after inoculation and the time between detectable levels of PrP in the brain and occurrence of first clinical signs was less than or equal to 6 months.

It was assumed that the population most likely to yield BSE cases would be fallen adult cattle. These are animals too ill to be slaughtered, wasting or dead animals, and animals that do not recover after treatment. Carcasses from such animals and all specified bovine offal are collected and processed at one particular Swiss rendering plant. Thus, samples were taken in this rendering plant by two veterinarians. To make certain that an infected animal could be traced back, all collection points and transporters of culled animals use special ear tags to identify these animals in addition to issuing an accompanying document.

The cohort of emergency slaughtered animals was expected to be the second most likely population to yield BSE cases, since these animals were ill or had an accident prior to slaughter.

Thus since January 1999 all fallen adult cattle and since March 1999 all adult cattle subject to emergency slaughter are being examined. In addition, a random sample of regularly slaughtered cattle are being examined. Samples are being taken by veterinarians responsible for meat inspection. Approximately 6000 fallen cattle will be sampled, about 3000 samples will be taken from animals subject to emergency slaughter; and approximately 7200 adult cattle will be sampled at routine slaughter-for a total of about 16000 animals to be examined in this study. Mandatory reporting of all clinically suspect animals (based on neurological symptoms) will go on concomitantly as usual.

All brain samples are being tested for the presence of PrP(Sc) using the Prionics-Check-Test; test-positive samples will be examined at the Swiss reference laboratory for animal TSE by means of histology and/or immunohistochemistry.

As of 30 June 1999, a total of about 3400 fallen animals, 1380 emergency-slaughtered animals, and 2765 routinely slaughtered cows have been examined. Ten of the fallen animals, three of the emergency slaughtered cows and one of the routinely slaughtered cows were found to be BSE-positive. Retrospective investigations of these cases demonstrated that only some of them showed signs typical for BSE. Most of them had displayed some signs of distress, unspecific disease or loss of productivity.

The results obtained so far clearly show that mandatory reporting of clinically suspect animals alone is not sufficient to derive a clear picture of the BSE status in a country, because such reporting depends mainly on subjective factors. It is obvious that by using a targeted surveillance scheme - such as the one outlined here - it is expected to detect more BSE cases than through mandatory reporting alone. This scheme, in concert with the mandatory reporting of clinically suspect animals, is considered to be a very good approach to further evaluate the status and monitor the evolution of the BSE epidemic in the cattle population.

We conclude that a targeted surveillance scheme, applied particularly to all fallen stock and emergency-slaughtered animals, is the most effective approach to detect a rare disease such as BSE on a country-wide scale.

Volver al índice